1. Field of the Invention
The present invention relates to an image reading device by which an image signal, recorded in a recording medium, is sensed using a line sensor, for example.
2. Description of the Related Art
Conventionally, in an image reading device that senses the image by using an imaging device such as a line sensor, there are several ways to detect a focal position at which an image to be read is focused. For example, an image signal is read by the line sensor, and is subjected to a filtering process using a low pass filter, which passes signals derived when a focusing lens is not in an in-focus condition, so that a filtered image signal is obtained. The filtered image signal is subtracted from the original image signal, and the resultant image signal obtained by the subtraction is integrated, so that the contrast of the image is obtained. The focal position is detected in accordance with the contrast.
Thus, a complicated calculation is needed in the filtering process to detect the focal position. Further, when an image signal is processed, it should be taken into consideration that frequency characteristics of the image signal differ depending upon an object to be read. Furthermore, if the image signal contains noise, since the noise is integrated together with the image signal or a phase of light is inverted due to the limit of performance of the lens, the peak value of the contrast may not be calculated with a high accuracy.